Conventionally, a step for manufacturing a sealed battery includes a leak testing step for checking the airtightness of a battery case for the purpose of, for example, prevention of degradation of battery performance caused by ingress of moisture into the battery case (see JP-A 2002-117901, for example).
JP-A 2002-117901 discloses a technique as follows:
First, a pouring nozzle is brought into contact with a lid of a battery can (a battery case), and is attached to a pouring hole.
Next, an electrolyte solution is poured into the battery can through the pouring nozzle, and a helium gas is introduced into the battery can.
Then, the pouring nozzle is detached from the pouring hole to seal the pouring hole with a laser welding means.
Finally, the sealed battery can is placed in a chamber for leak detection to perform a leak testing step. In the leak testing step, a determination is made with a helium-leak tester as to whether the helium gas leaks from the battery can.
The helium gas has molecular weight smaller than that of a gas inside the battery can. Therefore, as shown in FIG. 15, most of the helium gas introduced through the pouring hole stays in the vicinity of the pouring hole, and leaks to the outside of the battery can through the pouring hole before the leak testing step.
In other words, in the technique disclosed in JP-A 2002-117901, since a large amount of the helium gas leaks out before the leak testing step, a density of the helium gas inside the battery can is not maintained during the leak testing step, and consequently the density of the helium gas inside the battery can decreases in the leak testing step.
In the leak testing step, for example, a determination of the leak test is made by measuring an amount of the helium gas leaking from the battery can per unit time based on an output value of the helium-leak tester. As shown in a graph G11 in FIG. 16, if the density of the helium gas inside the battery can is low in the leak testing step, the amount of the helium gas leaking from the battery can per unit time decreases. Therefore, the output value of the helium-leak tester is wholly low.
As shown in FIG. 16, a threshold T1 of the leak testing step is set in consideration of the case where the density of the helium gas inside the battery can is low in the leak testing step. In other words, for example, the following value is set to the threshold T1: the output value of the helium-leak tester for the case of testing the battery can in which the density of the helium gas is low in the leak testing step and the leakage amount of the helium gas per unit time is a predetermined amount L.
As in the technique disclosed in JP-A 2002-117901, if the leakage amount of the helium gas before the leak testing step is large, the density of the helium gas inside the battery can varies widely during the leak testing step.
Therefore, in the case of testing the battery can in which a leakage amount L0 of the helium gas per unit time is slightly smaller than the predetermined amount L, if the density of the helium gas inside the battery can is larger, by the effect of the variation in the density of the helium gas, than that inside the battery can in which the leakage amount of the helium gas is the predetermined amount L, the output value of the helium-leak tester may exceed the threshold T1 with a relatively high probability (see the dot and a graph G12 in FIG. 16).
As mentioned above, in the technique disclosed in JP-A 2002-117901, a normal product may be erroneously determined to be a defective product with a relatively high probability (see an area R11 in FIG. 16 where an erroneous determination is made).
In other words, the technique disclosed in JP-A 2002-117901 may cause an increase in an erroneous determination rate.